Not infrequently, microfilms are prepared with the characters of original documents recorded thereon as oriented randomly, horizontally or vertically. Accordingly microfilm readers or reader-printers have heretofore been used in which an image rotation prism is interposed between the projection lens and the screen for rotating the image on the screen to orient the image as desired.
FIG. 1 is a diagram showing a microfilm reader having such image rotation means. The image on a microfilm M sandwiched between flat glass plates G is projected onto a screen 7 via a projection lens 2, an image rotation prism 3 and mirrors 4, 5, 6. The image rotation prism 3 is rotated, for example, by the mechanism disclosed in U.S. Pat. No. 3,907,418. The rotation of the prism 3 rotates the image on the screen 7 without moving the film M. The image rotation prism shown in FIG. 1 is a dove prism. Although prisms of the shape shown in FIG. 2a or 2b are also usable, dove prisms, which can be compacted, are used generally.
The projection lens system conventionally used in such a microfilm reader or reader-printer has its exit pupil positioned substantially at the center of the lens system and therefore has the drawback of requiring a large prism and forming deteriorated images when the field angle is wide, even if the prism is disposed close to the exit end of the lens system. This will be described in greater detail. The dove prism is optically equivalent to a plane-parallel glass plate which is positioned at an angle of inclination of 45.degree. with respect to the optical axis and has aberrations for converging rays. The degree of aberration for a bundle of rays incident on the prism surface in an inclined direction with respect to the surface differs from that for another bundle of rays which is within the same circular image space as the former bundle and which is incident on the prism surface in a less inclined direction. The degree of aberration (such as axial astigmatism) is in proportion to the length of bottom surface of the prism. It is accordingly desired that the prism 3 be of the smallest possible size, but with the conventional arrangement wherein the exit pupil of the projection lens 2 is positioned at or near the center of the lens, the rays passing through various points of the microfilm spread out widely in the vicinity of the prism 3 as illustrated. Thus, the prism 3 needs to be of large size to cover the spread of the rays.
The use of a small-sized prism can be realized by disposing the aperture stop for the projection lens system at the end of the system toward the screen 7 (i.e. the long conjugate side) as proposed in Published Examined Japanese Patent Applications SHO 47-35027 and SHO 47-35028.
However, when the stop is positioned in the space at the screen side, various other drawbacks result, e.g. increased curvature of field and increased astigmatism. Especially if the field angle is wide, it becomes difficult to correct coma. Further because there is a limitation to the combination of glass materials for simultaneously correcting axial chromatic aberration and aberration associated with the magnification, it is difficult to obtain overall correction for other aberrations. Furthermore, there arises the drawback that an attempt to fully correct aberrations requires a lens which is large-sized in its entirety.
The lens system proposed as above is still unable to fully overcome these drawbacks. The system is narrow in field angle, low in the brightness of the lens and large-sized. The size of the lens system is about 1.5 in terms of telescopic ratio which is a value obtained by dividing the distance from the image side end face of the lens to the film surface by the overall focal length f. The smaller this value, the more compact is the lens system.